How to Prevent Bagworms

One of the most destructive landscape pests is the bagworm. They are fond of eating Leyland cypresses, arborvitae, and cedars.

Bagworms are caterpillars that make a cocoon shaped like a diamond. They do not make webs like those of spiders and tent caterpillars.

Bagworm eggs hatch near the end of May or the first week in June. Though these little worms are only 1/8 of an inch long, they begin to test the strength of their silk by spinning a thread and hanging around. Spring breezes catch the worms and often blow them into Leyland cypresses, among other trees.

If a bagworm is lucky enough to land on a suitable host plant, then it spins a silk cocoon. This cocoon serves as protection from predators, sunlight, and pesticides. Foliage from the host plant is cleverly incorporated into the silk cocoon as a disguise. Bagworms were the original inventors of the ghillie suit.

As bagworms feed on foliage, they grow and enlarge their silk protection. Homeowners finally begin to spot these pests in August or September. By then, the feeding damage is very obvious and the caterpillars are difficult to kill. Since young caterpillars are easier to eliminate, you should apply an insecticide in late May or early June.

Products containing Bt (Bacillus thuringiensis), like DiPel or Worm Whipper, are not contact insecticides. They have to be eaten by young caterpillars. To apply, simply spray Bt on the foliage and let the baby bagworms nibble. This target specific, non-contact type of pesticide bypasses the protective cocoon and is safe to the applicator and surrounding environment. However, this method does not work well on mature caterpillars.

Contact insecticides like Sevin, Malathion and Orthene, while less safe for the surrounding environment, are also more effective when caterpillars are young. By the end of August or September, bagworms are nearly invincible and your only removal option is handpicking.

Handpicking bagworm cocoons from your trees is an option when the trees are small. However, Leyland cypresses can grow 50 feet tall, which makes this option nearly impossible. Once the cocoons have been picked from the trees, destroy them by burying them underground.

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